DE-C-1 163 784 discloses a process for rendering pyrogenic silica having silanol groups at its surface hydrophobic by treating with dimethyldichlorosilane in a cocurrent fluidized bed at 200.degree.-800.degree. C., preferably at 400.degree.-600.degree. C., in the presence of water vapor and under an inert gas, for example nitrogen.
Processes which have been employed heretofore to render pyrogenic silica hydrophobic have the disadvantage that the reaction has to be carried out at high temperatures. It is accordingly necessary to supply a great deal of energy in the form of heat. This increases inter alia the manufacturing costs of the hydrophobic product.
Therefore, it is an object of the present invention to provide a process for rendering silica hydrophobic at low temperatures.